/*
** $Id: lgc.c,v 2.133 2012/05/31 21:28:59 roberto Exp $
** Garbage Collector
** See Copyright Notice in lua.h
*/

#include <string.h>

#define lgc_c
#define LUA_CORE

#include "lua.h"

#include "ldebug.h"
#include "ldo.h"
#include "lfunc.h"
#include "lgc.h"
#include "lmem.h"
#include "lobject.h"
#include "lstate.h"
#include "lstring.h"
#include "ltable.h"
#include "ltm.h"

/*
** cost of sweeping one element (the size of a small object divided
** by some adjust for the sweep speed)
*/
#define GCSWEEPCOST ((sizeof(TString) + 4) / 4)

/* maximum number of elements to sweep in each single step */
#define GCSWEEPMAX (cast_int((GCSTEPSIZE / GCSWEEPCOST) / 4))

/* maximum number of finalizers to call in each GC step */
#define GCFINALIZENUM 4

/*
** macro to adjust 'stepmul': 'stepmul' is actually used like
** 'stepmul / STEPMULADJ' (value chosen by tests)
*/
#define STEPMULADJ 200

/*
** macro to adjust 'pause': 'pause' is actually used like
** 'pause / PAUSEADJ' (value chosen by tests)
*/
#define PAUSEADJ 200

/*
** standard negative debt for GC; a reasonable "time" to wait before
** starting a new cycle
*/
#define stddebtest(g, e) (-cast(l_mem, (e) / PAUSEADJ) * g->gcpause)
#define stddebt(g) stddebtest(g, gettotalbytes(g))

/*
** 'makewhite' erases all color bits plus the old bit and then
** sets only the current white bit
*/
#define maskcolors (~(bit2mask(BLACKBIT, OLDBIT) | WHITEBITS))
#define makewhite(g, x) \
  (gch(x)->marked = cast_byte((gch(x)->marked & maskcolors) | luaC_white(g)))

#define white2gray(x) resetbits(gch(x)->marked, WHITEBITS)
#define black2gray(x) resetbit(gch(x)->marked, BLACKBIT)

#define isfinalized(x) testbit(gch(x)->marked, FINALIZEDBIT)

#define checkdeadkey(n) lua_assert(!ttisdeadkey(gkey(n)) || ttisnil(gval(n)))

#define checkconsistency(obj) \
  lua_longassert(!iscollectable(obj) || righttt(obj))

#define markvalue(g, o)                \
  {                                    \
    checkconsistency(o);               \
    if (valiswhite(o))                 \
      reallymarkobject(g, gcvalue(o)); \
  }

#define markobject(g, t)               \
  {                                    \
    if ((t) && iswhite(obj2gco(t)))    \
      reallymarkobject(g, obj2gco(t)); \
  }

static void reallymarkobject(global_State *g, GCObject *o);

/*
** {======================================================
** Generic functions
** =======================================================
*/

/*
** one after last element in a hash array
*/
#define gnodelast(h) gnode(h, cast(size_t, sizenode(h)))

/*
** link table 'h' into list pointed by 'p'
*/
#define linktable(h, p) ((h)->gclist = *(p), *(p) = obj2gco(h))

/*
** if key is not marked, mark its entry as dead (therefore removing it
** from the table)
*/
static void removeentry(Node *n)
{
  lua_assert(ttisnil(gval(n)));
  if (valiswhite(gkey(n)))
    setdeadvalue(gkey(n)); /* unused and unmarked key; remove it */
}

/*
** tells whether a key or value can be cleared from a weak
** table. Non-collectable objects are never removed from weak
** tables. Strings behave as `values', so are never removed too. for
** other objects: if really collected, cannot keep them; for objects
** being finalized, keep them in keys, but not in values
*/
static int iscleared(global_State *g, const TValue *o)
{
  if (!iscollectable(o))
    return 0;
  else if (ttisstring(o))
  {
    markobject(g, rawtsvalue(o)); /* strings are `values', so are never weak */
    return 0;
  }
  else
    return iswhite(gcvalue(o));
}

/*
** barrier that moves collector forward, that is, mark the white object
** being pointed by a black object.
*/
void luaC_barrier_(lua_State *L, GCObject *o, GCObject *v)
{
  global_State *g = G(L);
  lua_assert(isblack(o) && iswhite(v) && !isdead(g, v) && !isdead(g, o));
  lua_assert(isgenerational(g) || g->gcstate != GCSpause);
  lua_assert(gch(o)->tt != LUA_TTABLE);
  if (keepinvariant(g))     /* must keep invariant? */
    reallymarkobject(g, v); /* restore invariant */
  else
  { /* sweep phase */
    lua_assert(issweepphase(g));
    makewhite(g, o); /* mark main obj. as white to avoid other barriers */
  }
}

/*
** barrier that moves collector backward, that is, mark the black object
** pointing to a white object as gray again. (Current implementation
** only works for tables; access to 'gclist' is not uniform across
** different types.)
*/
void luaC_barrierback_(lua_State *L, GCObject *o)
{
  global_State *g = G(L);
  lua_assert(isblack(o) && !isdead(g, o) && gch(o)->tt == LUA_TTABLE);
  black2gray(o); /* make object gray (again) */
  gco2t(o)->gclist = g->grayagain;
  g->grayagain = o;
}

/*
** barrier for prototypes. When creating first closure (cache is
** NULL), use a forward barrier; this may be the only closure of the
** prototype (if it is a "regular" function, with a single instance)
** and the prototype may be big, so it is better to avoid traversing
** it again. Otherwise, use a backward barrier, to avoid marking all
** possible instances.
*/
LUAI_FUNC void luaC_barrierproto_(lua_State *L, Proto *p, Closure *c)
{
  global_State *g = G(L);
  lua_assert(isblack(obj2gco(p)));
  if (p->cache == NULL)
  { /* first time? */
    luaC_objbarrier(L, p, c);
  }
  else
  {                         /* use a backward barrier */
    black2gray(obj2gco(p)); /* make prototype gray (again) */
    p->gclist = g->grayagain;
    g->grayagain = obj2gco(p);
  }
}

/*
** check color (and invariants) for an upvalue that was closed,
** i.e., moved into the 'allgc' list
*/
void luaC_checkupvalcolor(global_State *g, UpVal *uv)
{
  GCObject *o = obj2gco(uv);
  lua_assert(!isblack(o)); /* open upvalues are never black */
  if (isgray(o))
  {
    if (keepinvariant(g))
    {
      resetoldbit(o); /* see MOVE OLD rule */
      gray2black(o);  /* it is being visited now */
      markvalue(g, uv->v);
    }
    else
    {
      lua_assert(issweepphase(g));
      makewhite(g, o);
    }
  }
}

/*
** create a new collectable object (with given type and size) and link
** it to '*list'. 'offset' tells how many bytes to allocate before the
** object itself (used only by states).
*/
GCObject *luaC_newobj(lua_State *L, int tt, size_t sz, GCObject **list,
                      int offset)
{
  global_State *g = G(L);
  char *raw = cast(char *, luaM_newobject(L, novariant(tt), sz));
  GCObject *o = obj2gco(raw + offset);
  if (list == NULL)
    list = &g->allgc; /* standard list for collectable objects */
  gch(o)->marked = luaC_white(g);
  gch(o)->tt = tt;
  gch(o)->next = *list;
  *list = o;
  return o;
}

/* }====================================================== */

/*
** {======================================================
** Mark functions
** =======================================================
*/

/*
** mark an object. Userdata, strings, and closed upvalues are visited
** and turned black here. Other objects are marked gray and added
** to appropriate list to be visited (and turned black) later. (Open
** upvalues are already linked in 'headuv' list.)
*/
static void reallymarkobject(global_State *g, GCObject *o)
{
  lu_mem size;
  white2gray(o);
  switch (gch(o)->tt)
  {
  case LUA_TSHRSTR:
  case LUA_TLNGSTR:
  {
    size = sizestring(gco2ts(o));
    break; /* nothing else to mark; make it black */
  }
  case LUA_TUSERDATA:
  {
    Table *mt = gco2u(o)->metatable;
    markobject(g, mt);
    markobject(g, gco2u(o)->env);
    size = sizeudata(gco2u(o));
    break;
  }
  case LUA_TUPVAL:
  {
    UpVal *uv = gco2uv(o);
    markvalue(g, uv->v);
    if (uv->v != &uv->u.value) /* open? */
      return;                  /* open upvalues remain gray */
    size = sizeof(UpVal);
    break;
  }
  case LUA_TLCL:
  {
    gco2lcl(o)->gclist = g->gray;
    g->gray = o;
    return;
  }
  case LUA_TCCL:
  {
    gco2ccl(o)->gclist = g->gray;
    g->gray = o;
    return;
  }
  case LUA_TTABLE:
  {
    linktable(gco2t(o), &g->gray);
    return;
  }
  case LUA_TTHREAD:
  {
    // 2020/07/08 批注：
    // 这两句要结合着propagatemark()中的这一段：
    //case LUA_TTHREAD:
    //{
    //  lua_State *th = gco2th(o);
    //  g->gray = th->gclist; /* remove from 'gray' list */
    //  th->gclist = g->grayagain;
    //  g->grayagain = o; /* insert into 'grayagain' list */
    //  black2gray(o);
    //  size = traversestack(g, th);
    //  break;
    //}
    // 这样意思就很清晰了：这两句把g->gray这个链表暂存在o->gclist中，同时把链表的头指针赋值为o，当propagatemark
    // 中访问到该object时，再把g->gray还原回去.
    // 根据上面的解释，markroot()会把基础的节点依次串到g->gray中，类似table->thread->(g->gray)，然后propagatemark处理时，会依propagatemark次处理
    gco2th(o)->gclist = g->gray;
    g->gray = o;
    return;
  }
  case LUA_TPROTO:
  {
    gco2p(o)->gclist = g->gray;
    g->gray = o;
    return;
  }
  default:
    lua_assert(0);
    return;
  }
  gray2black(o);
  g->GCmemtrav += size;
}

/*
** mark metamethods for basic types
*/
static void markmt(global_State *g)
{
  int i;
  for (i = 0; i < LUA_NUMTAGS; i++)
    markobject(g, g->mt[i]);
}

/*
** mark all objects in list of being-finalized
*/
static void markbeingfnz(global_State *g)
{
  GCObject *o;
  for (o = g->tobefnz; o != NULL; o = gch(o)->next)
  {
    makewhite(g, o);
    reallymarkobject(g, o);
  }
}

/*
** mark all values stored in marked open upvalues. (See comment in
** 'lstate.h'.)
*/
static void remarkupvals(global_State *g)
{
  UpVal *uv;
  for (uv = g->uvhead.u.l.next; uv != &g->uvhead; uv = uv->u.l.next)
  {
    if (isgray(obj2gco(uv)))
      markvalue(g, uv->v);
  }
}

/*
** mark root set and reset all gray lists, to start a new
** incremental (or full) collection
*/
static void markroot(global_State *g)
{
  g->gray = g->grayagain = NULL;
  g->weak = g->allweak = g->ephemeron = NULL;
  markobject(g, g->mainthread);
  markvalue(g, &g->l_registry);
  markmt(g);
  markbeingfnz(g); /* mark any finalizing object left from previous cycle */
}

/* }====================================================== */

/*
** {======================================================
** Traverse functions
** =======================================================
*/

static void traverseweakvalue(global_State *g, Table *h)
{
  Node *n, *limit = gnodelast(h);
  /* if there is array part, assume it may have white values (do not
     traverse it just to check) */
  int hasclears = (h->sizearray > 0);
  for (n = gnode(h, 0); n < limit; n++)
  {
    checkdeadkey(n);
    if (ttisnil(gval(n))) /* entry is empty? */
      removeentry(n);     /* remove it */
    else
    {
      lua_assert(!ttisnil(gkey(n)));
      markvalue(g, gkey(n));                   /* mark key */
      if (!hasclears && iscleared(g, gval(n))) /* is there a white value? */
        hasclears = 1;                         /* table will have to be cleared */
    }
  }
  if (hasclears)
    linktable(h, &g->weak);      /* has to be cleared later */
  else                           /* no white values */
    linktable(h, &g->grayagain); /* no need to clean */
}

static int traverseephemeron(global_State *g, Table *h)
{
  int marked = 0;    /* true if an object is marked in this traversal */
  int hasclears = 0; /* true if table has white keys */
  int prop = 0;      /* true if table has entry "white-key -> white-value" */
  Node *n, *limit = gnodelast(h);
  int i;
  /* traverse array part (numeric keys are 'strong') */
  for (i = 0; i < h->sizearray; i++)
  {
    if (valiswhite(&h->array[i]))
    {
      marked = 1;
      reallymarkobject(g, gcvalue(&h->array[i]));
    }
  }
  /* traverse hash part */
  for (n = gnode(h, 0); n < limit; n++)
  {
    checkdeadkey(n);
    if (ttisnil(gval(n))) /* entry is empty? */
      removeentry(n);     /* remove it */
    else if (iscleared(g, gkey(n)))
    {                          /* key is not marked (yet)? */
      hasclears = 1;           /* table must be cleared */
      if (valiswhite(gval(n))) /* value not marked yet? */
        prop = 1;              /* must propagate again */
    }
    else if (valiswhite(gval(n)))
    { /* value not marked yet? */
      marked = 1;
      reallymarkobject(g, gcvalue(gval(n))); /* mark it now */
    }
  }
  if (prop)
    linktable(h, &g->ephemeron); /* have to propagate again */
  else if (hasclears)            /* does table have white keys? */
    linktable(h, &g->allweak);   /* may have to clean white keys */
  else                           /* no white keys */
    linktable(h, &g->grayagain); /* no need to clean */
  return marked;
}

static void traversestrongtable(global_State *g, Table *h)
{
  Node *n, *limit = gnodelast(h);
  int i;
  for (i = 0; i < h->sizearray; i++) /* traverse array part */
    markvalue(g, &h->array[i]);
  for (n = gnode(h, 0); n < limit; n++)
  { /* traverse hash part */
    checkdeadkey(n);
    if (ttisnil(gval(n))) /* entry is empty? */
      removeentry(n);     /* remove it */
    else
    {
      lua_assert(!ttisnil(gkey(n)));
      markvalue(g, gkey(n)); /* mark key */
      markvalue(g, gval(n)); /* mark value */
    }
  }
}

static lu_mem traversetable(global_State *g, Table *h)
{
  const char *weakkey, *weakvalue;
  const TValue *mode = gfasttm(g, h->metatable, TM_MODE);
  markobject(g, h->metatable);
  if (mode && ttisstring(mode) && /* is there a weak mode? */
      ((weakkey = strchr(svalue(mode), 'k')),
       (weakvalue = strchr(svalue(mode), 'v')),
       (weakkey || weakvalue)))
  {                         /* is really weak? */
    black2gray(obj2gco(h)); /* keep table gray */
    if (!weakkey)           /* strong keys? */
      traverseweakvalue(g, h);
    else if (!weakvalue) /* strong values? */
      traverseephemeron(g, h);
    else                         /* all weak */
      linktable(h, &g->allweak); /* nothing to traverse now */
  }
  else /* not weak */
    traversestrongtable(g, h);
  return sizeof(Table) + sizeof(TValue) * h->sizearray +
         sizeof(Node) * sizenode(h);
}

static int traverseproto(global_State *g, Proto *f)
{
  int i;
  if (f->cache && iswhite(obj2gco(f->cache)))
    f->cache = NULL; /* allow cache to be collected */
  markobject(g, f->source);
  for (i = 0; i < f->sizek; i++) /* mark literals */
    markvalue(g, &f->k[i]);
  for (i = 0; i < f->sizeupvalues; i++) /* mark upvalue names */
    markobject(g, f->upvalues[i].name);
  for (i = 0; i < f->sizep; i++) /* mark nested protos */
    markobject(g, f->p[i]);
  for (i = 0; i < f->sizelocvars; i++) /* mark local-variable names */
    markobject(g, f->locvars[i].varname);
  return sizeof(Proto) + sizeof(Instruction) * f->sizecode +
         sizeof(Proto *) * f->sizep +
         sizeof(TValue) * f->sizek +
         sizeof(int) * f->sizelineinfo +
         sizeof(LocVar) * f->sizelocvars +
         sizeof(Upvaldesc) * f->sizeupvalues;
}

static lu_mem traverseCclosure(global_State *g, CClosure *cl)
{
  int i;
  for (i = 0; i < cl->nupvalues; i++) /* mark its upvalues */
    markvalue(g, &cl->upvalue[i]);
  return sizeCclosure(cl->nupvalues);
}

static lu_mem traverseLclosure(global_State *g, LClosure *cl)
{
  int i;
  markobject(g, cl->p);               /* mark its prototype */
  for (i = 0; i < cl->nupvalues; i++) /* mark its upvalues */
    markobject(g, cl->upvals[i]);
  return sizeLclosure(cl->nupvalues);
}

static lu_mem traversestack(global_State *g, lua_State *th)
{
  StkId o = th->stack;
  if (o == NULL)
    return 1; /* stack not completely built yet */
  for (; o < th->top; o++)
    markvalue(g, o);
  if (g->gcstate == GCSatomic)
  {                                        /* final traversal? */
    StkId lim = th->stack + th->stacksize; /* real end of stack */
    for (; o < lim; o++)                   /* clear not-marked stack slice */
      setnilvalue(o);
  }
  return sizeof(lua_State) + sizeof(TValue) * th->stacksize;
}

/*
** traverse one gray object, turning it to black (except for threads,
** which are always gray).
** 2020/07/08 批注：
** 为什么叫 propagatemark ?
** 因为是从当前 gray 对象出发, 去搜索所有被它们引用的对象, 再从所有被找到的对象出发,
** 再找所有被它们引用的对象. 这样持续下去直到所有被直接或间接引用到的对象都被找到. 所以叫 propagate (传播)
** 该函数调用一次处理一个灰色的对象
*/
static void propagatemark(global_State *g)
{
  lu_mem size;
  GCObject *o = g->gray;
  lua_assert(isgray(o));
  gray2black(o);
  switch (gch(o)->tt)
  {
  case LUA_TTABLE:
  {
    Table *h = gco2t(o);
    g->gray = h->gclist; /* remove from 'gray' list */ /* 将reallymarkobject函数中赋值到h中的g->gray再还原回去 */
    size = traversetable(g, h);
    break;
  }
  case LUA_TLCL:
  {
    LClosure *cl = gco2lcl(o);
    g->gray = cl->gclist; /* remove from 'gray' list */
    size = traverseLclosure(g, cl);
    break;
  }
  case LUA_TCCL:
  {
    CClosure *cl = gco2ccl(o);
    g->gray = cl->gclist; /* remove from 'gray' list */
    size = traverseCclosure(g, cl);
    break;
  }
  case LUA_TTHREAD:
  {
    lua_State *th = gco2th(o);
    g->gray = th->gclist; /* remove from 'gray' list */
    th->gclist = g->grayagain;
    g->grayagain = o; /* insert into 'grayagain' list */
    black2gray(o);
    size = traversestack(g, th);
    break;
  }
  case LUA_TPROTO:
  {
    Proto *p = gco2p(o);
    g->gray = p->gclist; /* remove from 'gray' list */
    size = traverseproto(g, p);
    break;
  }
  default:
    lua_assert(0);
    return;
  }
  g->GCmemtrav += size;
}

static void propagateall(global_State *g)
{
  while (g->gray)
    propagatemark(g);
}

static void propagatelist(global_State *g, GCObject *l)
{
  lua_assert(g->gray == NULL); /* no grays left */
  g->gray = l;
  propagateall(g); /* traverse all elements from 'l' */
}

/*
** retraverse all gray lists. Because tables may be reinserted in other
** lists when traversed, traverse the original lists to avoid traversing
** twice the same table (which is not wrong, but inefficient)
*/
static void retraversegrays(global_State *g)
{
  GCObject *weak = g->weak; /* save original lists */
  GCObject *grayagain = g->grayagain;
  GCObject *ephemeron = g->ephemeron;
  g->weak = g->grayagain = g->ephemeron = NULL;
  propagateall(g); /* traverse main gray list */
  propagatelist(g, grayagain);
  propagatelist(g, weak);
  propagatelist(g, ephemeron);
}

static void convergeephemerons(global_State *g)
{
  int changed;
  do
  {
    GCObject *w;
    GCObject *next = g->ephemeron; /* get ephemeron list */
    g->ephemeron = NULL;           /* tables will return to this list when traversed */
    changed = 0;
    while ((w = next) != NULL)
    {
      next = gco2t(w)->gclist;
      if (traverseephemeron(g, gco2t(w)))
      {                  /* traverse marked some value? */
        propagateall(g); /* propagate changes */
        changed = 1;     /* will have to revisit all ephemeron tables */
      }
    }
  } while (changed);
}

/* }====================================================== */

/*
** {======================================================
** Sweep Functions
** =======================================================
*/

/*
** clear entries with unmarked keys from all weaktables in list 'l' up
** to element 'f'
*/
static void clearkeys(global_State *g, GCObject *l, GCObject *f)
{
  for (; l != f; l = gco2t(l)->gclist)
  {
    Table *h = gco2t(l);
    Node *n, *limit = gnodelast(h);
    for (n = gnode(h, 0); n < limit; n++)
    {
      if (!ttisnil(gval(n)) && (iscleared(g, gkey(n))))
      {
        setnilvalue(gval(n)); /* remove value ... */
        removeentry(n);       /* and remove entry from table */
      }
    }
  }
}

/*
** clear entries with unmarked values from all weaktables in list 'l' up
** to element 'f'
*/
static void clearvalues(global_State *g, GCObject *l, GCObject *f)
{
  for (; l != f; l = gco2t(l)->gclist)
  {
    Table *h = gco2t(l);
    Node *n, *limit = gnodelast(h);
    int i;
    for (i = 0; i < h->sizearray; i++)
    {
      TValue *o = &h->array[i];
      if (iscleared(g, o)) /* value was collected? */
        setnilvalue(o);    /* remove value */
    }
    for (n = gnode(h, 0); n < limit; n++)
    {
      if (!ttisnil(gval(n)) && iscleared(g, gval(n)))
      {
        setnilvalue(gval(n)); /* remove value ... */
        removeentry(n);       /* and remove entry from table */
      }
    }
  }
}

static void freeobj(lua_State *L, GCObject *o)
{
  switch (gch(o)->tt)
  {
  case LUA_TPROTO:
    luaF_freeproto(L, gco2p(o));
    break;
  case LUA_TLCL:
  {
    luaM_freemem(L, o, sizeLclosure(gco2lcl(o)->nupvalues));
    break;
  }
  case LUA_TCCL:
  {
    luaM_freemem(L, o, sizeCclosure(gco2ccl(o)->nupvalues));
    break;
  }
  case LUA_TUPVAL:
    luaF_freeupval(L, gco2uv(o));
    break;
  case LUA_TTABLE:
    luaH_free(L, gco2t(o));
    break;
  case LUA_TTHREAD:
    luaE_freethread(L, gco2th(o));
    break;
  case LUA_TUSERDATA:
    luaM_freemem(L, o, sizeudata(gco2u(o)));
    break;
  case LUA_TSHRSTR:
    G(L)->strt.nuse--;
    /* go through */
  case LUA_TLNGSTR:
  {
    luaM_freemem(L, o, sizestring(gco2ts(o)));
    break;
  }
  default:
    lua_assert(0);
  }
}

#define sweepwholelist(L, p) sweeplist(L, p, MAX_LUMEM)
static GCObject **sweeplist(lua_State *L, GCObject **p, lu_mem count);

/*
** sweep the (open) upvalues of a thread and resize its stack and
** list of call-info structures.
*/
static void sweepthread(lua_State *L, lua_State *L1)
{
  if (L1->stack == NULL)
    return;                          /* stack not completely built yet */
  sweepwholelist(L, &L1->openupval); /* sweep open upvalues */
  luaE_freeCI(L1);                   /* free extra CallInfo slots */
  /* should not change the stack during an emergency gc cycle */
  if (G(L)->gckind != KGC_EMERGENCY)
    luaD_shrinkstack(L1);
}

/*
** sweep at most 'count' elements from a list of GCObjects erasing dead
** objects, where a dead (not alive) object is one marked with the "old"
** (non current) white and not fixed.
** In non-generational mode, change all non-dead objects back to white,
** preparing for next collection cycle.
** In generational mode, keep black objects black, and also mark them as
** old; stop when hitting an old object, as all objects after that
** one will be old too.
** When object is a thread, sweep its list of open upvalues too.
*/
static GCObject **sweeplist(lua_State *L, GCObject **p, lu_mem count)
{
  global_State *g = G(L);
  int ow = otherwhite(g);
  int toclear, toset; /* bits to clear and to set in all live objects */
  int tostop;         /* stop sweep when this is true */
  if (isgenerational(g))
  {                           /* generational mode? */
    toclear = ~0;             /* clear nothing */
    toset = bitmask(OLDBIT);  /* set the old bit of all surviving objects */
    tostop = bitmask(OLDBIT); /* do not sweep old generation */
  }
  else
  {                        /* normal mode */
    toclear = maskcolors;  /* clear all color bits + old bit */
    toset = luaC_white(g); /* make object white */
    tostop = 0;            /* do not stop */
  }
  while (*p != NULL && count-- > 0)
  {
    GCObject *curr = *p;
    int marked = gch(curr)->marked;
    if (isdeadm(ow, marked))
    {                       /* is 'curr' dead? */
      *p = gch(curr)->next; /* remove 'curr' from list */
      freeobj(L, curr);     /* erase 'curr' */
    }
    else
    {
      if (testbits(marked, tostop))
        return NULL; /* stop sweeping this list */
      if (gch(curr)->tt == LUA_TTHREAD)
        sweepthread(L, gco2th(curr)); /* sweep thread's upvalues */
      /* update marks */
      gch(curr)->marked = cast_byte((marked & toclear) | toset);
      p = &gch(curr)->next; /* go to next element */
    }
  }
  return (*p == NULL) ? NULL : p;
}

/*
** sweep a list until a live object (or end of list)
*/
static GCObject **sweeptolive(lua_State *L, GCObject **p, int *n)
{
  GCObject **old = p;
  int i = 0;
  do
  {
    i++;
    p = sweeplist(L, p, 1);
  } while (p == old);
  if (n)
    *n += i;
  return p;
}

/* }====================================================== */

/*
** {======================================================
** Finalization
** =======================================================
*/

static void checkSizes(lua_State *L)
{
  global_State *g = G(L);
  if (g->gckind != KGC_EMERGENCY)
  {                                        /* do not change sizes in emergency */
    int hs = g->strt.size / 2;             /* half the size of the string table */
    if (g->strt.nuse < cast(lu_int32, hs)) /* using less than that half? */
      luaS_resize(L, hs);                  /* halve its size */
    luaZ_freebuffer(L, &g->buff);          /* free concatenation buffer */
  }
}

static GCObject *udata2finalize(global_State *g)
{
  GCObject *o = g->tobefnz; /* get first element */
  lua_assert(isfinalized(o));
  g->tobefnz = gch(o)->next; /* remove it from 'tobefnz' list */
  gch(o)->next = g->allgc;   /* return it to 'allgc' list */
  g->allgc = o;
  resetbit(gch(o)->marked, SEPARATED); /* mark that it is not in 'tobefnz' */
  lua_assert(!isold(o));               /* see MOVE OLD rule */
  if (!keepinvariant(g))               /* not keeping invariant? */
    makewhite(g, o);                   /* "sweep" object */
  return o;
}

static void dothecall(lua_State *L, void *ud)
{
  UNUSED(ud);
  luaD_call(L, L->top - 2, 0, 0);
}

static void GCTM(lua_State *L, int propagateerrors)
{
  global_State *g = G(L);
  const TValue *tm;
  TValue v;
  setgcovalue(L, &v, udata2finalize(g));
  tm = luaT_gettmbyobj(L, &v, TM_GC);
  if (tm != NULL && ttisfunction(tm))
  { /* is there a finalizer? */
    int status;
    lu_byte oldah = L->allowhook;
    int running = g->gcrunning;
    L->allowhook = 0;            /* stop debug hooks during GC metamethod */
    g->gcrunning = 0;            /* avoid GC steps */
    setobj2s(L, L->top, tm);     /* push finalizer... */
    setobj2s(L, L->top + 1, &v); /* ... and its argument */
    L->top += 2;                 /* and (next line) call the finalizer */
    status = luaD_pcall(L, dothecall, NULL, savestack(L, L->top - 2), 0);
    L->allowhook = oldah;   /* restore hooks */
    g->gcrunning = running; /* restore state */
    if (status != LUA_OK && propagateerrors)
    { /* error while running __gc? */
      if (status == LUA_ERRRUN)
      { /* is there an error object? */
        const char *msg = (ttisstring(L->top - 1))
                              ? svalue(L->top - 1)
                              : "no message";
        luaO_pushfstring(L, "error in __gc metamethod (%s)", msg);
        status = LUA_ERRGCMM; /* error in __gc metamethod */
      }
      luaD_throw(L, status); /* re-throw error */
    }
  }
}

/*
** move all unreachable objects (or 'all' objects) that need
** finalization from list 'finobj' to list 'tobefnz' (to be finalized)
*/
static void separatetobefnz(lua_State *L, int all)
{
  global_State *g = G(L);
  GCObject **p = &g->finobj;
  GCObject *curr;
  GCObject **lastnext = &g->tobefnz;
  /* find last 'next' field in 'tobefnz' list (to add elements in its end) */
  while (*lastnext != NULL)
    lastnext = &gch(*lastnext)->next;
  while ((curr = *p) != NULL)
  { /* traverse all finalizable objects */
    lua_assert(!isfinalized(curr));
    lua_assert(testbit(gch(curr)->marked, SEPARATED));
    if (!(all || iswhite(curr))) /* not being collected? */
      p = &gch(curr)->next;      /* don't bother with it */
    else
    {
      l_setbit(gch(curr)->marked, FINALIZEDBIT); /* won't be finalized again */
      *p = gch(curr)->next;                      /* remove 'curr' from 'finobj' list */
      gch(curr)->next = *lastnext;               /* link at the end of 'tobefnz' list */
      *lastnext = curr;
      lastnext = &gch(curr)->next;
    }
  }
}

/*
** if object 'o' has a finalizer, remove it from 'allgc' list (must
** search the list to find it) and link it in 'finobj' list.
*/
void luaC_checkfinalizer(lua_State *L, GCObject *o, Table *mt)
{
  global_State *g = G(L);
  if (testbit(gch(o)->marked, SEPARATED) || /* obj. is already separated... */
      isfinalized(o) ||                     /* ... or is finalized... */
      gfasttm(g, mt, TM_GC) == NULL)        /* or has no finalizer? */
    return;                                 /* nothing to be done */
  else
  { /* move 'o' to 'finobj' list */
    GCObject **p;
    GCheader *ho = gch(o);
    if (g->sweepgc == &ho->next)
    { /* avoid removing current sweep object */
      lua_assert(issweepphase(g));
      g->sweepgc = sweeptolive(L, g->sweepgc, NULL);
    }
    /* search for pointer pointing to 'o' */
    for (p = &g->allgc; *p != o; p = &gch(*p)->next)
    { /* empty */
    }
    *p = ho->next;        /* remove 'o' from root list */
    ho->next = g->finobj; /* link it in list 'finobj' */
    g->finobj = o;
    l_setbit(ho->marked, SEPARATED); /* mark it as such */
    if (!keepinvariant(g))           /* not keeping invariant? */
      makewhite(g, o);               /* "sweep" object */
    else
      resetoldbit(o); /* see MOVE OLD rule */
  }
}

/* }====================================================== */

/*
** {======================================================
** GC control
** =======================================================
*/

#define sweepphases \
  (bitmask(GCSsweepstring) | bitmask(GCSsweepudata) | bitmask(GCSsweep))

/*
** enter first sweep phase (strings) and prepare pointers for other
** sweep phases.  The calls to 'sweeptolive' make pointers point to an
** object inside the list (instead of to the header), so that the real
** sweep do not need to skip objects created between "now" and the start
** of the real sweep.
** Returns how many objects it sweeped.
*/
static int entersweep(lua_State *L)
{
  global_State *g = G(L);
  int n = 0;
  g->gcstate = GCSsweepstring;
  lua_assert(g->sweepgc == NULL && g->sweepfin == NULL);
  /* prepare to sweep strings, finalizable objects, and regular objects */
  g->sweepstrgc = 0;
  g->sweepfin = sweeptolive(L, &g->finobj, &n);
  g->sweepgc = sweeptolive(L, &g->allgc, &n);
  return n;
}

/*
** change GC mode
*/
void luaC_changemode(lua_State *L, int mode)
{
  global_State *g = G(L);
  if (mode == g->gckind)
    return; /* nothing to change */
  if (mode == KGC_GEN)
  { /* change to generational mode */
    /* make sure gray lists are consistent */
    luaC_runtilstate(L, bitmask(GCSpropagate));
    g->GCestimate = gettotalbytes(g);
    g->gckind = KGC_GEN;
  }
  else
  { /* change to incremental mode */
    /* sweep all objects to turn them back to white
       (as white has not changed, nothing extra will be collected) */
    g->gckind = KGC_NORMAL;
    entersweep(L);
    luaC_runtilstate(L, ~sweepphases);
  }
}

/*
** call all pending finalizers
*/
static void callallpendingfinalizers(lua_State *L, int propagateerrors)
{
  global_State *g = G(L);
  while (g->tobefnz)
  {
    resetoldbit(g->tobefnz);
    GCTM(L, propagateerrors);
  }
}

void luaC_freeallobjects(lua_State *L)
{
  global_State *g = G(L);
  int i;
  separatetobefnz(L, 1); /* separate all objects with finalizers */
  lua_assert(g->finobj == NULL);
  callallpendingfinalizers(L, 0);
  g->currentwhite = WHITEBITS; /* this "white" makes all objects look dead */
  g->gckind = KGC_NORMAL;
  sweepwholelist(L, &g->finobj); /* finalizers can create objs. in 'finobj' */
  sweepwholelist(L, &g->allgc);
  for (i = 0; i < g->strt.size; i++) /* free all string lists */
    sweepwholelist(L, &g->strt.hash[i]);
  lua_assert(g->strt.nuse == 0);
}

static l_mem atomic(lua_State *L)
{
  global_State *g = G(L);
  l_mem work = -g->GCmemtrav; /* start counting work */
  GCObject *origweak, *origall;
  lua_assert(!iswhite(obj2gco(g->mainthread)));
  markobject(g, L); /* mark running thread */
  /* registry and global metatables may be changed by API */
  markvalue(g, &g->l_registry);
  markmt(g); /* mark basic metatables */
  /* remark occasional upvalues of (maybe) dead threads */
  remarkupvals(g);
  propagateall(g);      /* propagate changes */
  work += g->GCmemtrav; /* stop counting (do not (re)count grays) */
  /* traverse objects caught by write barrier and by 'remarkupvals' */
  retraversegrays(g);
  work -= g->GCmemtrav; /* restart counting */
  convergeephemerons(g);
  /* at this point, all strongly accessible objects are marked. */
  /* clear values from weak tables, before checking finalizers */
  clearvalues(g, g->weak, NULL);
  clearvalues(g, g->allweak, NULL);
  origweak = g->weak;
  origall = g->allweak;
  work += g->GCmemtrav;  /* stop counting (objects being finalized) */
  separatetobefnz(L, 0); /* separate objects to be finalized */
  markbeingfnz(g);       /* mark objects that will be finalized */
  propagateall(g);       /* remark, to propagate `preserveness' */
  work -= g->GCmemtrav;  /* restart counting */
  convergeephemerons(g);
  /* at this point, all resurrected objects are marked. */
  /* remove dead objects from weak tables */
  clearkeys(g, g->ephemeron, NULL); /* clear keys from all ephemeron tables */
  clearkeys(g, g->allweak, NULL);   /* clear keys from all allweak tables */
  /* clear values from resurrected weak tables */
  clearvalues(g, g->weak, origweak);
  clearvalues(g, g->allweak, origall);
  g->currentwhite = cast_byte(otherwhite(g)); /* flip current white */
  work += g->GCmemtrav;                       /* complete counting */
  return work;                                /* estimate of memory marked by 'atomic' */
}

static lu_mem singlestep(lua_State *L)
{
  global_State *g = G(L);
  switch (g->gcstate)
  {
  case GCSpause:
  {
    g->GCmemtrav = 0; /* start to count memory traversed */
    if (!isgenerational(g))
      markroot(g); /* start a new collection */
    /* in any case, root must be marked at this point */
    lua_assert(!iswhite(obj2gco(g->mainthread)) && !iswhite(gcvalue(&g->l_registry)));
    g->gcstate = GCSpropagate;
    return g->GCmemtrav;
  }
  case GCSpropagate:
  {
    if (g->gray)
    {
      lu_mem oldtrav = g->GCmemtrav;
      propagatemark(g);
      return g->GCmemtrav - oldtrav; /* memory traversed in this step */
    }
    else
    { /* no more `gray' objects */
      lu_mem work;
      int sw;
      g->gcstate = GCSatomic;       /* finish mark phase */
      g->GCestimate = g->GCmemtrav; /* save what was counted */
      ;
      work = atomic(L);      /* add what was traversed by 'atomic' */
      g->GCestimate += work; /* estimate of total memory traversed */
      sw = entersweep(L);
      return work + sw * GCSWEEPCOST;
    }
  }
  case GCSsweepstring:
  {
    int i;
    for (i = 0; i < GCSWEEPMAX && g->sweepstrgc + i < g->strt.size; i++)
      sweepwholelist(L, &g->strt.hash[g->sweepstrgc + i]);
    g->sweepstrgc += i;
    if (g->sweepstrgc >= g->strt.size) /* no more strings to sweep? */
      g->gcstate = GCSsweepudata;
    return i * GCSWEEPCOST;
  }
  case GCSsweepudata:
  {
    if (g->sweepfin)
    {
      g->sweepfin = sweeplist(L, g->sweepfin, GCSWEEPMAX);
      return GCSWEEPMAX * GCSWEEPCOST;
    }
    else
    {
      g->gcstate = GCSsweep;
      return 0;
    }
  }
  case GCSsweep:
  {
    if (g->sweepgc)
    {
      g->sweepgc = sweeplist(L, g->sweepgc, GCSWEEPMAX);
      return GCSWEEPMAX * GCSWEEPCOST;
    }
    else
    {
      /* sweep main thread */
      GCObject *mt = obj2gco(g->mainthread);
      sweeplist(L, &mt, 1);
      checkSizes(L);
      g->gcstate = GCSpause; /* finish collection */
      return GCSWEEPCOST;
    }
  }
  default:
    lua_assert(0);
    return 0;
  }
}

/*
** advances the garbage collector until it reaches a state allowed
** by 'statemask'
*/
void luaC_runtilstate(lua_State *L, int statesmask)
{
  global_State *g = G(L);
  while (!testbit(statesmask, g->gcstate))
    singlestep(L);
}

static void generationalcollection(lua_State *L)
{
  global_State *g = G(L);
  if (g->GCestimate == 0)
  {                                   /* signal for another major collection? */
    luaC_fullgc(L, 0);                /* perform a full regular collection */
    g->GCestimate = gettotalbytes(g); /* update control */
  }
  else
  {
    lu_mem estimate = g->GCestimate;
    luaC_runtilstate(L, ~bitmask(GCSpause)); /* run complete cycle */
    luaC_runtilstate(L, bitmask(GCSpause));
    if (gettotalbytes(g) > (estimate / 100) * g->gcmajorinc)
      g->GCestimate = 0; /* signal for a major collection */
  }
  luaE_setdebt(g, stddebt(g));
}

static void incstep(lua_State *L)
{
  global_State *g = G(L);
  l_mem debt = g->GCdebt;
  int stepmul = g->gcstepmul;
  if (stepmul < 40)
    stepmul = 40; /* avoid ridiculous low values */
  /* convert debt from Kb to 'work units' (avoid zero debt and overflows) */
  debt = (debt / STEPMULADJ) + 1;
  debt = (debt < MAX_LMEM / stepmul) ? debt * stepmul : MAX_LMEM;
  do
  {                              /* always perform at least one single step */
    lu_mem work = singlestep(L); /* do some work */
    debt -= work;
  } while (debt > -GCSTEPSIZE && g->gcstate != GCSpause);
  if (g->gcstate == GCSpause)
    debt = stddebtest(g, g->GCestimate); /* pause until next cycle */
  else
    debt = (debt / stepmul) * STEPMULADJ; /* convert 'work units' to Kb */
  luaE_setdebt(g, debt);
}

/*
** performs a basic GC step
*/
void luaC_forcestep(lua_State *L)
{
  global_State *g = G(L);
  int i;
  if (isgenerational(g))
    generationalcollection(L);
  else
    incstep(L);
  /* run a few finalizers (or all of them at the end of a collect cycle) */
  for (i = 0; g->tobefnz && (i < GCFINALIZENUM || g->gcstate == GCSpause); i++)
    GCTM(L, 1); /* call one finalizer */
}

/*
** performs a basic GC step only if collector is running
*/
void luaC_step(lua_State *L)
{
  global_State *g = G(L);
  if (g->gcrunning)
    luaC_forcestep(L);
  else
    luaE_setdebt(g, -GCSTEPSIZE); /* avoid being called too often */
}

/*
** performs a full GC cycle; if "isemergency", does not call
** finalizers (which could change stack positions)
*/
void luaC_fullgc(lua_State *L, int isemergency)
{
  global_State *g = G(L);
  int origkind = g->gckind;
  int someblack = keepinvariant(g);
  lua_assert(origkind != KGC_EMERGENCY);
  if (isemergency) /* do not run finalizers during emergency GC */
    g->gckind = KGC_EMERGENCY;
  else
  {
    g->gckind = KGC_NORMAL;
    callallpendingfinalizers(L, 1);
  }
  if (someblack)
  { /* may there be some black objects? */
    /* must sweep all objects to turn them back to white
       (as white has not changed, nothing will be collected) */
    entersweep(L);
  }
  /* finish any pending sweep phase to start a new cycle */
  luaC_runtilstate(L, bitmask(GCSpause));
  /* run entire collector */
  luaC_runtilstate(L, ~bitmask(GCSpause));
  luaC_runtilstate(L, bitmask(GCSpause));
  if (origkind == KGC_GEN)
  { /* generational mode? */
    /* generational mode must always start in propagate phase */
    luaC_runtilstate(L, bitmask(GCSpropagate));
  }
  g->gckind = origkind;
  luaE_setdebt(g, stddebt(g));
  if (!isemergency) /* do not run finalizers during emergency GC */
    callallpendingfinalizers(L, 1);
}

/* }====================================================== */
